This invention relates in general to electron guns for television cathode ray picture tubes, and more specifically to an improved extended field main focus lens for such guns.
The focus lens provided according to the present invention represents a new and useful improvement of the electron gun having an extended field electrostatic focus lens described and fully claimed in U.S. Pat. No. 3,995,194 to Blacker et al, and assigned to the assignee of the present invention. The subject Blacker et al patent, hereinafter referred to as the "'194" patent and incorporated herein by reference, entails a television cathode ray tube having associated therewith a power supply for developing discrete supply voltages. A general purpose electron gun is provided for receiving supply voltages from the power supply to produce a sharply focused beam of electrons at the cathode ray tube screen. The gun according to the '194 patent comprises associated cathode means and grid means for producing a beam of electrons, and a novel focus lens means. The focus lens means receives electrons from the cathode means and the predetermined pattern of voltages from the power supply and comprises at least three electrodes for establishing a single, continuous electrostatic focusing field characterized by having an axial potential distribution which, at all times during tube operation, decreases smoothly and monotonically from a relatively intermediate potential to a relatively low potential spatially located at a lens intermediate position, and then increases smoothly, directly and monotonically from the relatively low potential to a relatively high potential. This novel axial potential distribution is represented diagrammatically by FIG. 1.
The novel focus lens set forth by the '194 patent takes advantage of the low aberration produced by the extended field lens described and claimed in U.S. Pat. No. 3,985,253 to J. Schwartz et al. The extended field provides reduced beam spot size even at high beam currents, and thus offers markedly improved picture resolution in all screen sizes.
The structure and relationship of an electron gun of the type described and a cathode ray picture tube, and the prior art means for supplying operating voltages to the combination, are shown by FIG. 2. The primary components of a typical color picture tube 10 comprise an evacuated envelope including a neck 12, a funnel 14 and a faceplace 16. On an inner surface of the faceplate 16 are deposited a multiplicity of cathodoluminescent phosphor target elements 18 comprising a pattern of groups of red-light-emitting, green-light-emitting, and blue-light-emitting dots or stripes. A foraminated electrode 20 called a "shadow mask", is used in the tube for color selection. Base 22 provides entrance means for a plurality of electrically conductive lead-in pins 24.
The electron gun 26, illustrated schematically, is disposed within neck 12 substantially as shown. Gun 26 is commonly installed in axial alignment with a center line X--X of picture tube 10. Gun 26 emits electron beams 28 to selectively activate target elements 18.
Power supply 30, also shown schematically, provides voltages for operation of the cathode ray tube and its electron gun. To supply the required potentials, a special voltage divider circuit is typically incorporated into the power supply circuit. Power supply 30 may supply relatively low voltages in the one to eight kilovolt range through one or more leads represented schematically by 32, which enter the envelope of tube 10 through the plurality of lead-in pins 24 in base 22. Power supply 30 also supplies selected intermediate voltages to the focus electrodes of electron gun 26, voltages typically in the range of eight to fifteen kilovolts or higher; these voltages are indicated as being supplied to the electrodes within the envelope of tube 12 by way of lead-in pins 24 through lead 33. The relatively high voltage for electron gun operation; this is, a voltage typically in the range of twenty-five to thirty-five kilovolts for excitation of the accelerating anodes is shown as being indirectly supplied to gun 26 through lead 34, which is connected to anode button 36. Anode button 36 in turn introduces the high voltage through the glass envelope of funnel 14, making contact with a thin, electrically conductive coating 38 disposed on the internal surface of funnel 14, and part-way into neck 12. The anode electrode of gun 26 receives the relatively high anode voltage through a plurality of metallic gun centering springs 40 extending from gun 26 and in physical contact with inner conductive coating 38.
The requirement to introduce the relatively low voltage of about 8 kilovolts together with the relatively intermediate voltage of about 12 kilovolts into the cathode ray tube envelope 10 through lead-in pins 24 has limited the applicability of the electron gun according to the '194 patent. The routing of two such comparatively high voltages through the lead-in pins has typically required an elaborate socket, indicated schematically by 42 in combination with base 22 and associated lead-in pins 24. The close adjacency of lead-in pins 24, and the wide range of potentials thereon has made it necessary to devise tube socket-base combinations capable of shielding the lead-in pins one from the other. Isolative means have included insulative barriers or walls molded as part of the socket and base to extend prospective arc paths. Sockets have also comprised non-destructive arcing paths in their structure, and arc-quenching means embodied in the socket and base combination. It has also often been necessary to introduce potting compounds into the tube base to eliminate arc-prone air paths between leads. This complexity of the socket and base combination adds to manufacturing costs.
The requirement for two base-introduced high-voltage potentials presents a further disadvantage in that an electron gun requiring such potentials can be used only in receivers having special power supply circuits adapted specifically for their use. Thus, the use of the gun of the '194 patent, despite its marked benefits, has been denied to a large segment of the television set population which lacks the necessary power supply and base-socket provisions. This denial encompasses otherwise excellent "older" receivers produced prior to the inception of the gun of the '194 patent. An example is the type of television chassis designed for a tube incorporating a gun with a bipotential lens which requires only two voltages in the high range--a single voltage of, typically, 8 or 9 kilovolts, introduced through the base, and a voltage of 30 kilovolts introduced through the funnel.